Present day engine components must be manufactured more simply at significantly reduced costs while achieving superior results in order for engine manufacturers to remain competitive. Unfortunately, port liners that have become less complicated have either failed to produce superior heat insulation capabilities or have become less durable, increasing associated replacement costs.
The heat-insulated port liner for a device composed of a cast metal disclosed in U.S. Pat. No. 4,676,064 by Yoshinori Narita et. al. on Jun. 30, 1987 includes a tubular port liner composed of a ceramic material, a first covering layer disposed on the outer surface of the liner and composed of refractory fibers, and a second covering layer disposed on the outer surface of the first covering layer and composed of a metal having a melting point not lower than the melting point of the cast metal. The port liner is made from a material having a low coefficient of thermal expansion and high thermal resistance, such as, aluminum titanate. Unfortunately, no range is given for the coefficient of thermal expansion needed for the port liner used with a cast aluminum cylinder head. It is well known that the melting point of aluminum is lower than that of cast iron and that aluminum titanate can be effectively used with molten aluminum. However, aluminum titanate would decompose if exposed to molten cast iron due to the additional stresses. Therefore, an aluminum cylinder head can utilize a port liner made from a material having a higher coefficient of thermal expansion than a cast iron cylinder head without damage to the port liner during the casting process. The port liner disclosed by Narita would be destroyed during the casting process if used with a cast iron cylinder head. Additionally, since the first covering layer is unsupported, settling of the refractory fibers occurs when the fibers are exposed to typical engine vibration experienced during operation. This settling effect limits the effectiveness of the insulation and may lead to the destruction of the entire insulation layer. Once destroyed, the insulation would be free to disintegrate and enter the exhaust passage.
A method and apparatus for insulating the exhaust passage of an internal combustion engine is disclosed in U.S. Pat. No. 4,206,598 by Vemulapalli D. Rao on Jun. 10, 1980. A three-zone liner assembly is provided with an outer zone comprised of a room temperature vulcanizing silicone sleeve, an inner zone comprised of a stamped and seam welded high strength Al--Cr-steel alloy, and an intermediate zone consisting of a ceramic wool mat. The intermediate zone consisting of the ceramic wool mat of insulation is encased within the seam welded inner zone of metal protecting the insulation from damage. However, if the weld fails, the insulation is subjected to possible damage which, as with Narita, would cause disintegration of the insulation and destruction of the entire insulation layer.
The present invention is directed to overcoming the problems as set forth above.